The present invention relates to an apparatus and method for detecting the location of an electrical discharge in the machining zone of a travelling wire EDM apparatus.
Travelling wire EDM apparatus are designed to cut an electrode workpiece, by machining electrical discharges, by means of an electrode wire. A pulse generator, providing the machining electrical discharges, is connected across the electrode workpiece. The connection to the electrode wire is generally effected by a pair of electrical conductors supplying the machining current, one conductor being connected to the electrode wire on one side of the machining zone and the other on the other side.
Connecting the pulse generator to the electrode wire by way of a pair of electrical conductors is a necessary expedient for keeping under control the excessive thermal stress imposed upon the electrode wire due to heating caused by the electrical discharges themselves and by the resistive heating of the wire resulting from the flow of machining current through the wire. The heating of the wire, if remaining unchecked, may exceed acceptable limits and cause rupture of the wire, for example if the electrical discharges are concentrated at a specific location along the portion of the wire in the machining zone. Monitoring the concentration of the elecrical discharges by way of detecting the precise location of each electrical discharge permits to prevent electrode wire rupture.
One known method for detecting the location of an electrical discharge which is disclosed in published Japanese patent application No. 53/64899 consists in measuring the resistance of the electrode wire between the location of the electrical discharge and one of the contacts supplying machining current to the electrode wire. Such a method presents the inconvenience of measuring a voltage drop on a portion of the wire, which is a measurement that is very inaccurate due to the low impedance of the circuit in which the measurement is effected.
Another method, also disclosed in the same Japanese patent application publication consists in measuring the voltage unbalance of a Wheatstone bridge, one of the branches of the bridge consisting of the length of wire between the two contacts supplying machining current to the wire, each contact being located on one side of the machining zone. Such a method is practical only if all the electrical discharges occur at the same current level. Otherwise, as is for example the case when the pulse generator is of the relaxation type, which is often the type of pulse generators used in travelling wire EDM apparatus, the intensity of the electrical discharges varies as a function of the triggering voltage of the discharges, and the amplitude of voltage unbalance of the bridge varies as a function of the intensity. Under such circumstances, the known method becomes useless.